1. Field of the Invention
The present general inventive concept relates to an illumination system capable of eliminating speckle of laser beams emitted from a laser light source and a projection system employing the illumination system, and more particularly, to an illumination system and a projection system which can effectively reduce or eliminate laser speckle by moving a diffractive optical element and time-averaging partial beams.
2. Description of the Related Art
Since projection systems including a laser light source have a wide color gamut, they have excellent collimation and high light efficiency. However, such projection systems have speckle due to coherent illumination. Speckle results from an interferential interaction of coherent waves that are scattered from a diffusely reflecting surface. Such speckle constitutes a significant cause of deterioration in image resolution and quality.
A conventional projector for eliminating speckle disclosed in U.S. Pat. No. 6,606,173 B2 is illustrated in FIGS. 1 and 2.
Referring to FIG. 1, the conventional projector includes a laser light source 1, a beam expander 2, a holographic diffuser 3, a first field lens 4, and a second field lens 5, all of which are aligned on the same axis. A Gaussian laser beam emitted from the laser light source 1 is divided by the beam expander 2, the holographic diffuser 3, the first field lens 4, and the second field lens 5 into beams with uniform rectangular sections.
Referring to FIGS. 1 and 2, the beam emitted from the laser light source 1 is expanded into a collimated beam by the beam expander 2. The collimated laser beam is divided by the holographic diffuser 3 into a plurality of rectangular beams. Each of the rectangular beams is diffused with a horizontal angle θhoriz and a vertical angle θvert. The horizontal angle θhoriz and the vertical angle θvert can be adjusted according to a diffraction pattern of the holographic diffuser 3. The rectangular beams are converged by the first field lens 4 such that the beams with a Gaussian distribution are transformed into uniform beams. The rectangular beams are adjusted to have shapes corresponding to a micro display system. Beams having passed through the first field lens 4 are refracted into telecentric beams by the second field lens 5.
Since the collimated beam is transformed into the rectangular beams having a uniform intensity distribution by the holographic diffuser 3 and the rectangular beams are converged, laser speckle is reduced. However, when the laser speckle is reduced using only the holographic diffuser 3, the laser speckle is not completely eliminated and image quality deteriorates.